Random-incidence scattering coefficients obtained by measurement in reverberation chambers (ISO 17497-1) for a particular sine-shaped surface are compared with theoretical results. Holford-Urusovskii's method and a hybrid plane wave decomposition - FEM method are applied to an infinite sine-shaped surface having the same profile, while the 3D BEM gives numerical results for the finite-size surface. Perfect rigidity (Neumann boundary condition) is assumed in all computations. The validity of the computations is compared and discussed with regard to the approximations involved. Finally, the measured and calculated values of the scattering coefficients are compared. A good correspondence is found, in particular concerning the overall dependence on frequency. Small deviations have however been observed, at low as well as at high frequencies. They will be explained by uncertainties in the measurements and discretisation errors in the calculations.